Most electronic devices such as, for example, computers, televisions, radio receivers and amplifiers include electrical circuits having electronic circuit boards. In many of these electronic devices the circuit boards comprise a dielectric substrate having a conductor path formed thereon. With respect to many of these prior art circuit boards the dielectric substrate may comprise alumina (Al.sub.2 O.sub.3) or beryllium oxide (BeO) and the conductor path may be formed on the substrate utilizing such metals as titanium (Ti), molybdenum (Mo) and manganese (Mn). Such conductor paths are generally formed upon the dielectric substrates by first producing a conductor paste and then applying the paste to the dielectric substrate utilizing conventional application techniques such as screen printing, spraying or brushing. The coated dielectric substrate is then heated in an oven so as to sinter and bond the metallic constituents of the paste to the dielectric substrate thereby forming a conductor path upon the dielectric substrate Attached to the conductor path may be various electronic components such as, for example, capacitors, diodes, rectifiers and various types of semiconductor devices. Such electronic components, depending on the particular metal(s) used to form the conductor path, may be attached utilizing various bonding techniques such as brazing, soldering, wire bonding or tab bonding.
During the operation or use of electronic devices having circuit boards the electronic components of the circuit boards generate heat. In many applications, the removal of the heat generated by the electronic components is of paramount importance. Specifically, the presence of excessive heat and the corresponding elevated temperatures that result from excessive heat build-up in the electronic components of a circuit board can temporarily alter or permanently damage the electrical characteristics of the electronic components.
One method of preventing excessive heat build-up in electronic circuits is to construct the circuit board utilizing a dielectric substrate having a high thermal conductivity. Such a dielectric substrate serves to dissipate the heat generated by the electrical components thereby helping to prevent the heat from altering or otherwise detrimentally affecting the electronic components of the circuit board. In addition to being a good thermal conductor, preferably the dielectric also displays a thermal coefficient of expansion that is closely related to silicon (Si), the material from which many electronic components are produced. This allows the dielectric substrate and the electronic components to expand at similar rates as heat is being generated by the electronic components thereby minimizing the possibility of an electrical break or short forming in the circuit board.
A dielectric material that displays excellent thermal conductivity and a thermal coefficient of expansion similar to that of Si is aluminum nitride (AlN). Thus, from the standpoint of helping to avoid excessive heat build-up and minimizing the possibility of an electrical break or short, aluminum nitride is a desirable material for the construction of circuit boards. However, in the prior art when a circuit board is produced utilizing aluminum nitride, many times the bond formed between the aluminum nitride substrate and the conductor path has been less than ideal. More particularly, some such prior art circuit boards have displayed adhesion strengths of less than about 5,000 psi between-thealuminum nitride substrate and the conductor path. This low bond strength has been found to be somewhat unacceptable for many applications. More particularly, when the conductor path separates from the aluminum nitride substrate due to poor adhesion between the conductor path and the substrate, electrical breaks or shorts are likely to develop in the circuit board. Generally, adhesion strengths in excess of 8,000 psi are preferred.
Some prior art circuit boards constructed with aluminum nitride have displayed acceptable adhesion strengths between the substrate and the conductor. Such conductor paths have been produced utilizing titanium nitride (TiN), Mo and tungsten (W). However, such prior art circuit boards have required during their production very high firing temperatures (e.g., temperatures in excess of 1800.degree. C.) in order to create a suitable bond between the conductor path and the substrate.
Other prior art circuit boards that display acceptable adhesion strengths between the aluminum nitride substrate and the conductor are those wherein the conductor is formed utilizing silver (Ag), palladium (Pd) or gold (Au). However, when attaching electronic components to circuit boards employing these particular conductor metals, it is not possible to utilize high temperature bonding techniques such as brazing.